The retention sites and residence times for inhaled particles deposited in the nonciliated pulmonary regions are important factors in the pathogenesis of major occupational respiratory diseases. This study is directed toward the quantitative characterization of the pathways, transport rates, and storage sites for respirable particles with defined size and shape and concentration. Particle retention within the thorax and tracheal lymph nodes following aerosol inhalation will be measured for at least 8 months in 60 rabbits by the external in vivo detection of radionuclides within the particles and for ferromagnetic particles, by remanent magnetism. These tests will provide a basis for estimating population variability and provide a baseline for each individual animal in the subsequent serial sacrifice experiments to determine translocations within the thorax, measurement of tissue burdens, the degree of particle ingestion by alveolar macrophages, the rate of particle accumulation in dust foci, and the proportions within the various cell types and the extracellular depots.